The present invention relates generally to plasma torches and, in particular, to a pilot arc circuit which provides improved pilot arc stability and reduced parts wear.
Plasma torches, also known as electric arc torches, are commonly used for cutting and welding metal workpieces by directing a plasma consisting of ionized gas particles toward the workpiece. A typical plasma torch has a torch tip spaced apart from and surrounding an electrode to define a channel or gap into which a plasma gas is supplied.
As is well known in the art, a pilot arc is initiated inside the torch head by applying a sufficiently high voltage across the channel when a stream of gas is flowing therethrough in the direction of an orifice in the torch tip. The torch tip constitutes a relatively positive potential anode, and the electrode, which has a relatively negative potential, operates as a cathode. The high voltage causes a spark to jump the gap between the electrode and the torch tip, thereby creating an electrically charged low impedance path in the plasma gas stream. Thus, the spark heats the gas, causes it to ionize and establishes a pilot arc current between the electrode and the inner surface of the torch tip.
The high voltage which initiates the spark may be generated by either a DC pulse (e.g., capacitive discharge circuits), an AC pulse (e.g., spark gap circuits) or a steady state DC supply, as disclosed in U.S. Pat. No. 5,235,162 to Nourbakhsh. Alternatively, the high voltage spark circuit may be eliminated in favor of a contact start method wherein the electrode and torch tip are brought in contact to initiate current flow and then separated to create the pilot arc.
The pilot arc may also be characterized by the nature and duration of the pilot arc current as follows: (1) a steady DC current of approximately constant level; (2) a DC current pulsing between a higher "pulse" level and a lower "background" level of longer duration; (3) a "blown out pilot" wherein a cycle of initiating the pilot arc for a short duration, extinguishing it and immediately reinitiating it for another short duration is repeated until the cutting arc is established or the start signal is removed; and (4) a single pulse similar to the "blown out pilot" except it does not repeat until the start signal is reapplied. U.S. Pat. No. 5,416,297 to Luo et al. is directed to this latter type of pilot arc, which is generally limited to mechanized cutting applications. Some plasma torch systems limit the duration of the pilot arc by shutting it off after a predetermined time if the cutting arc has not been established.
Once the pilot arc is established between the electrode and the inner surface of the torch tip, the force of the plasma gas flowing through the channel moves the pilot arc toward the tip orifice until it extends from the electrode to the outer surface of the torch tip through the orifice. As soon as the pilot arc has been blown through the tip orifice, the torch is ready to perform a cutting or welding operation. Since the impedance of the workpiece to ground is lower than the impedance of the torch tip to ground, the pilot arc will transfer from the torch tip to the workpiece when the torch is brought sufficiently close to the workpiece. Accordingly, the pilot arc jumps or transfers from the tip to the workpiece so that the workpiece becomes the anode, and the "transferred arc" performs the cutting or welding operation. The transferred arc, which is also commonly referred to as the "main" or "cutting" arc, generally continues until the start signal is removed or the torch is moved away from the workpiece.
Conventional plasma torches include circuitry for controlling the operation of the torch. Examples of such circuitry are disclosed in U.S. Pat. Nos. 5,170,030 to Solley et al. and 5,530,220 to Tatham, both of which are assigned to the assignee of the present invention and specifically incorporated herein by reference. Typically, the torch circuitry will include a pilot arc circuit in which the electrode and tip are coupled with a power source and a main arc circuit in which the electrode, tip and workpiece are coupled with a power source. The pilot arc circuit is usually disconnected upon establishment of the main arc. Sometimes, the same power source is used for both the pilot arc circuit and the main arc circuit. For example, U.S. Pat. No. 5,530,220 discloses a current regulated power circuit which supplies power for pilot arc initiation, pilot arc transfer and arc maintenance during operation on the workpiece.
There are a number of well-known problems and challenges associated with pilot arc initiation and transfer in plasma torches. One such problem is maintaining the pilot arc current at a level sufficient to initiate and maintain a stable pilot arc without causing excessive damage to the tip and electrode. Because the load voltage changes rapidly from about 10-40 volts (when the pilot arc is formed inside the torch head) to about 100-200 volts (after the pilot arc has been blown through the tip orifice), the current regulator cannot keep up and the pilot arc current level therefore droops to some extent below its peak level. If the pilot arc current droops below a minimum current sufficient to maintain the pilot arc, the arc will be unstable, resulting in a sputtering pilot which may go out, which may be discontinuous and/or which may be difficult to transfer. If the pilot arc goes out, the arc must be reinitiated which causes further damage to the torch. On the other hand, if the peak level of the pilot arc current is too high, the torch will sustain unnecessary damage since the wear on the tip and electrode increases as the level of the pilot arc current increases. Existing plasma torches that maintain the pilot arc current at a level high enough to avoid a noncontinuous or "sputtering" pilot arc suffer from excessive parts wear as a result of their relatively high average pilot arc current.